Coherent detection of optical signals offers a number of advantages with respect to the conventional technique of intensity modulation/direct detection. For example, coherent detection can offer .about. 10 to 20 dB improvement in receiver sensitivity while permitting alternative modulation formats, such as frequency-shift keying (FSK) and phase-shift keying (PSK). Particularly, these alternative modulation formats are important since direct intensity modulation of semiconductor lasers leads to chirp, which limits transmission distances through a fiber.
Homodyne detection offers practical advantages, such as minimum required receiver bandwidth, reduced receiver thermal noise and fully linear signal processing, which facilitate the attainment of near-quantum-limited sensitivity at high bit-rates. Also, multi-channel homodyne systems may employ a higher channel-packing density than their heterodyne counterparts. In theory, homodyne detection of phase-shift keyed optical signals has been shown to offer the best receiver sensitivity among all the binary signaling techniques employing single-bit decisions. However, implementation of BPSK homodyne receivers has not been straightforward because of the difficulty in phase-locking a local oscillator signal to a received optical signal that contains BPSK modulated data.
It is well known that phase-locking can be achieved either by means of a phase-locked loop or through injection locking. Due to the prohibitively high signal power levels required for injection-locking, utilizing a phase-locked loop has become the design of choice. Due to its ease of implementation, most phase-locked loop receivers employ a linear loop configuration with pilot-carrier BPSK, as opposed to a nonlinear loop (decision-driven loop or Costas loop) with suppressed-carrier BPSK.
The advantages of homodyne detection are especially important at multigigabit-per-second data rates. For application to linear optical phase-locked loops, however, present homodyne receivers have difficulties in providing a low-noise, dc-coupled phase error signal while amplifying multigigabit-per second data signals.